One of the key points in the book Natural Capitalism is that we are "liquidating our natural capital and calling it income". This is a way of stating that we are using up the earth's resources too fast in the language of economics. It is part of the book's argument that capitalism is not following its own rules.
The Telegraph recently reported on a UN report that tries to cost the services nature provides (also a theme in Natural Capitalism) - the current system seems to treat these services as free and unlimited a lot of the time. The estimate was £40 trillion - equal (interestingly) to the combined income of all the world's nations.